Untreated PTSD elevates risk of concurrent alcohol and drug dependence (>50% and >30% respectively), which in turn elevates risk of HIV sexual-risk behavior (HSB). A meta-analysis of HIV prevention, found few interventions address risk factors such as trauma exposure, mental health, or SUD and interventions which failed to do so had poorer condom use outcomes and were less effective for those with PTSD. From a public health perspective addressing PTSD and substance use disorders (SUD) is an important but overlooked means of preventing HIV/STI. American Indian (AI) communities are at disproportionate risk for untreated PTSD, SUD and HIV/STI risk. Self-medication of PTSD through substance use leads to development of SUDs, and exacerbates HSB. Therefore, treatment of PTSD that also decreases substance use in individuals may prevent the initiation or relapse of HIV risk behaviors. Similarly, directly intervening to reduce substance use via increasing motivation to decrease use and enhance coping strategies should prevent SUD development, and improve HSB and PTSD. Thus addressing PTSD or substance use may effectively prevent HSB. Built on an 8-year community-based participatory partnership, in full collaboration with the Tribal Nation, this study proposes a 5-year two-arm randomized comparative effectiveness trial to evaluate prevention of HIV/STI sexual risk behavior by directly addressing PTSD or substance use. We will evaluate Narrative Exposure Therapy (NET) versus Motivational Interviewing with Skills Training (MIST) in HIV/STI prevention. AI elder and community leaders have insisted that we can keep providing substance use or sexual risk programs but until we get to the underlying cause - trauma - then we'll keep replacing one means of avoidance for another. This study will help address this community-generated clinical and empirical question. The overall goal is to compare two evidence based treatments, each addressing a different HSB causal pathway. NET addresses PTSD preventing SUD and HSB. MIST addresses substance misuse preventing SUD and HSB. Aim 1: To examine a PTSD versus a substance use-focused intervention to prevent HSB. We will conduct a two-arm randomized comparative effectiveness trial comparing NET (n=100) to MIST (n=100) among AI men and women with PTSD symptoms. We will evaluate the effect of reductions in PTSD severity and substance use (e.g., frequency, quantity, intentions, relapse, and initiation of use) on HSB (e.g., number of partners, sex while using substances, unprotected sex, and sex with high-risk partners) at post, 6, and 12-month follow-up. Aim 2: To examine theoretical mediators of treatment on HSB outcomes including emotional regulation, cognitive reappraisals, coping self-efficacy, communication skills, assertiveness, and community connectiveness.